1. Field of the Invention
The present invention relates to a power supply apparatus and, more particularly, to a power supply apparatus which converts a power supply voltage into a target voltage using a combination of series regulator and switching regulator.
2. Description of the Related Art
In battery-driven portable equipment such as a portable telephone and a personal digital assistant (PDA), measures for extending battery life are taken. For example, the equipment may be placed in a sleep state or a standby state while a user is not using the equipment. In the equipment of this type, efficient voltage conversion is enabled by switchably putting into use a series regulator such as a low drop out (LDO) regulator and a switching regulator such as a DC/DC converter. In a low-load state such as a standby state, power consumption is controlled by putting a LDO regulator of a low power consumption type into use. When the load is increased while the equipment is being used, efficiency is improved by putting into use a DC/DC converter.
However, when a LDO regulator and a DC/DC converter are turned on and off in a mutually exclusive manner, the output voltage varies during a switching operation, preventing a stable supply of the voltage. Measures should be taken to control a voltage variation during a switching operation to a minimum level.
Japanese Laid-Open Patent Application 2003-9515 discloses a power supply system provided with a series regulator and a switching regulator for controlling a power supply output voltage at a constant level, and switchably putting into use the series regulator and the switching regulator in accordance with the magnitude of load current. There is provided a period of time in which both the series regulator and the switching regulator are operated while switching takes place.
By providing a period of time in which a series regulator and a switching regulator are concurrently operated while switching from a series regulator to a switching regulator takes place, an undershoot or an overshoot, in which the output voltage temporarily diverges from a target voltage at start-up of the switching regulator, occurs. This is because the series regulator and the switching regulator share a feedback point in a feedback system. When a transient variation in the output voltage occurs while switching takes place, it is impossible to output a stable converted voltage in a time span that includes the switching operation.
Related Art List
JPA laid open 2003-9515